In recent years, in order to meet a rapidly increasing demand for information communication, practical utilization of optical transmitters capable of long distance transmission, high speed transmission, and high density transmission is required. However, in an optical transmission system that performs long distance transmission such as a submarine cable system, there has been a problem that communication performance deteriorates due to transmission penalty accumulated along the length of an optical fiber of a communication path.
In an optical fiber through which a light signal of a single wavelength is transmitted, amplified spontaneous emission (ASE) noise generated in an erbium doped fiber amplifier (EDFA), nonlinear effect caused by the intensity of light propagating in a single fiber, and wavelength dispersion that causes different optical frequencies to proceed at different group velocities, etc. are generation cause of penalty. Moreover, in a wavelength division multiplexing (WDM) system where a plurality of different optical wavelengths exist in the same fiber, inter-wavelength crosstalk caused by a nonlinear refractive index of the fiber can be the penalty.
In the case of high-speed transmission in the optical transmission system, it is effective to increase data rate per wavelength, but increasing the data rate leads to intersymbol interference and causes degradation of a received eye pattern. Distortion of a received waveform is influenced by the configuration of communication lines and the shape of a communication pulse.
As a method for reducing such penalty, it is effective to perform pulse intensity modulation of various known formats synchronized with data symbols, such as RZ (Return-to-Zero) format and CS-RZ (Carrier-Suppressed Return-to-Zero) format, and such pulse intensity modulation is performed by connecting a plurality of optical modulators in cascade (see, for example, patent reference 1 and patent reference 2).
For example, when Mach-Zehnder LiNbO3 external modulators (hereinafter referred to as “Mach-Zehnder modulator”) are used in an optical transmitter having a plurality of optical modulators which are connected in cascade, it is necessary for each of the optical modulators to control the bias voltage of the optical modulator according to a modulation format (see, for example, patent reference 3).